Over the past decade, advantages of and advances in network computing have encouraged tremendous growth of computer networks, which has in turn spurred more advances, growth and advantages. With this growth, however, dislocations and bottlenecks have occurred in utilizing conventional network devices. For example, a CPU of a computer connected to a network may spend an increasing proportion of its time processing network communications, leaving less time available for other work. In particular, demands for moving file data between the network and a storage unit of the computer, such as a disk drive, have accelerated. Conventionally such data is divided into packets for transportation over the network, with each packet encapsulated in layers of control information that are processed one layer at a time by the CPU of the receiving computer. Although the speed of CPUs has constantly increased, this protocol processing of network messages such as file transfers can consume most of the available processing power of the fastest commercially available CPU.
This situation may be even more challenging for a network file server whose primary function is to store and retrieve files, on its attached disk or tape drives, by transferring file data over the network. As networks and databases have grown, the volume of information stored at such servers has exploded, exposing limitations of such server-attached storage. In addition to the above-mentioned problems of protocol processing by the host CPU, limitations of parallel data channels such as conventional small computer system interface (SCSI) interfaces have become apparent as storage needs have increased. For example, parallel SCSI interfaces restrict the number of storage devices that can be attached to a server and the distance between the storage devices and the server.
As noted in the book by Tom Clark entitled “Designing Storage Area Networks,” (copyright 1999) incorporated by reference herein, one solution to the limits of server-attached parallel SCSI storage devices involves attaching other file servers to an existing local area network (LAN) in front of the network server. This network-attached storage (NAS) allows access to the NAS file servers from other servers and clients on the network, but may not increase the storage capacity dedicated to the original network server. Conversely, NAS may increase the protocol processing required by the original network server, since that server may need to communicate with the various NAS file servers. In addition, each of the NAS file servers may in turn be subject to the strain of protocol processing and the limitations of storage interfaces.
Storage area networking (SAN) provides another solution to the growing need for file transfer and storage over networks, by replacing daisy-chained SCSI storage devices with a network of storage devices connected behind a server. Instead of conventional network standards such as Ethernet or Fast Ethernet, SANs deploy an emerging networking standard called Fibre Channel (FC). Due to its relatively recent introduction, however, many commercially available FC devices are incompatible with each other. Also, a FC network may dedicate bandwidth for communication between two points on the network, such as a server and a storage unit, the bandwidth being wasted when the points are not communicating.
NAS and SAN as known today can be differentiated according to the form of the data that is transferred and stored. NAS devices generally transfer data files to and from other file servers or clients, whereas device level blocks of data may be transferred over a SAN. For this reason, NAS devices conventionally include a file system for converting between files and blocks for storage, whereas a SAN may include storage devices that do not have such a file system.
Alternatively, NAS file servers can be attached to an Ethernet-based network dedicated to a server, as part of an Ethernet SAN. Marc Farley further states, in the book “Building Storage Networks,” (copyright 2000) incorporated by reference herein, that it is possible to run storage protocols over Ethernet, which may avoid Fibre Channel incompatibility issues. Increasing the number of storage devices connected to a server by employing a network topology such as SAN, however, increases the amount of protocol processing that must be performed by that server. As mentioned above, such protocol processing already strains the most advanced servers.
An example of conventional processing of a network message such as a file transfer illustrates some of the processing steps that slow network data storage. A network interface card (NIC) typically provides a physical connection between a host and a network or networks, as well as providing media access control (MAC) functions that allow the host to access the network or networks. When a network message packet sent to the host arrives at the NIC, MAC layer headers for that packet are processed and the packet undergoes cyclical redundancy checking (CRC) in the NIC. The packet is then sent across an input/output (I/O) bus such as a peripheral component interconnect (PCI) bus to the host, and stored in host memory. The CPU then processes each of the header layers of the packet sequentially by running instructions from the protocol stack. This requires a trip across the host memory bus initially for storing the packet and then subsequent trips across the host memory bus for sequentially processing each header layer. After all the header layers for that packet have been processed, the payload data from the packet is grouped in a file cache with other similarly-processed payload packets of the message. The data is reassembled by the CPU according to the file system as file blocks for storage on a disk or disks. After all the packets have been processed and the message has been reassembled as file blocks in the file cache, the file is sent, in blocks of data that may be each built from a few payload packets, back over the host memory bus and the I/O bus to host storage for long term storage on a disk, typically via a SCSI bus that is bridged to the I/O bus.
Alternatively, for storing the file on a SAN, the reassembled file in the file cache is sent in blocks back over the host memory bus and the I/O bus to an I/O controller configured for the SAN. For the situation in which the SAN is a FC network, a specialized FC controller is provided which can send the file blocks to a storage device on the SAN according to Fibre Channel Protocol (FCP). For the situation in which the file is to be stored on a NAS device, the file may be directed or redirected to the NAS device, which processes the packets much as described above but employs the CPU, protocol stack and file system of the NAS device, and stores blocks of the file on a storage unit of the NAS device.
Thus, a file that has been sent to a host from a network for storage on a SAN or NAS connected to the host typically requires two trips across an I/O bus for each message packet of the file. In addition, control information in header layers of each packet may cross the host memory bus repeatedly as it is temporarily stored, processed one layer at a time, and then sent back to the I/O bus. Retrieving such a file from storage on a SAN in response to a request from a client also conventionally requires significant processing by the host CPU and file system.